Indo-Western Pacific Climate Variability: ENSO Forcing and Internal Dynamics in a Tropical Pacific Pacemaker Simulation

Abstract

El Niño–Southern Oscillation (ENSO) peaks in boreal winter but its impact on Indo-western Pacific climate persists for another two seasons. Key ocean–atmosphere interaction processes for the ENSO effect are investigated using the Pacific Ocean–Global Atmosphere (POGA) experiment with a coupled general circulation model, where tropical Pacific sea surface temperature (SST) anomalies are restored to follow observations while the atmosphere and oceans are fully coupled elsewhere. The POGA shows skills in simulating the ENSO-forced warming of the tropical Indian Ocean and an anomalous anticyclonic circulation pattern over the northwestern tropical Pacific in the post–El Niño spring and summer. The 10-member POGA ensemble allows decomposing Indo-western Pacific variability into the ENSO forced and ENSO-unrelated (internal) components. Internal variability is comparable to the ENSO forcing in magnitude and independent of ENSO amplitude and phase. Random internal variability causes apparent decadal modulations of ENSO correlations over the Indo-western Pacific, which are high during epochs of high ENSO variance. This is broadly consistent with instrumental observations over the past 130 years as documented in recent studies. Internal variability features a sea level pressure pattern that extends into the north Indian Ocean and is associated with coherent SST anomalies from the Arabian Sea to the western Pacific, suggestive of ocean–atmosphere coupling.

ENSO-Unrelated Variability in Indo–Northwest Pacific Climate: Regional Coupled Ocean–Atmospheric Feedback

“…The ENSO-unrelated IPOC mode shows a strong resemblance to the observed counterpart except for the SST anomalies over the tropical southwestern IO (Figs. 1, 4), consistent with previous findings (Kosaka et al 2013;Wang et al 2018). Both aPOGA and cPOGA show positive SLP anomalies over the NWP, accompanied with an AAC and suppressed precipitation.…”

“…Whereas the IPOC is closely tied to ENSO in observations, recent model studies show that regional positive feedbacks can sustain the IPOC without ENSO forcing (e.g., Kosaka et al 2013;Wang et al 2018). Indeed, by linearly removing ENSO in observations (Wang et al 2018), or in partially coupled models without ENSO forcing (Kosaka et al 2013), an ENSO-unrelated IPOC mode emerges, resembling the original IPOC and supported by regional coupling processes. Such ENSO-unrelated, internal variability makes up a large portion of the interannual variability (Wang et al 2018) and can causes uncertainties in Indo-NWP climate prediction (Ma et al 2017).…”

“…Indeed, by linearly removing ENSO in observations (Wang et al 2018), or in partially coupled models without ENSO forcing (Kosaka et al 2013), an ENSO-unrelated IPOC mode emerges, resembling the original IPOC and supported by regional coupling processes. Such ENSO-unrelated, internal variability makes up a large portion of the interannual variability (Wang et al 2018) and can causes uncertainties in Indo-NWP climate prediction (Ma et al 2017). The complex nature of Indo-NWP summer climate variability and its potential social impact call for a better understanding of the internal variabilities, specifically, what roles does the ocean-atmosphere coupling play in supporting the ENSO-unrelated IPOC and how it is different from the ENSO-forced IPOC?…”

Intensification and Northward extension of Northwest Pacific anomalous anticyclone in El Niño decaying mid-summer: an energetic perspective

“…The fundamental works of the last twenty years successfully explain the formation and maintenance of AAC on the summer seasonal mean timescale, mainly focusing on the role of SST anomalies in three tropical oceans. Nevertheless, the AAC is not only a mode tied to anomalous SST forcing, but also could arise from the atmospheric internal dynamic processes unrelated to SST variability (Kosaka et al 2013;Zhou et al 2018;Wang et al 2018). Recent studies illustrated that the AAC could extract kinetic energy (KE) from background mean flow via barotropic energy conversion in the NWP confluence zone (Hu et al 2019;Wang et al 2020), implying that the impact of SST anomalies on the AAC becomes more efficient under the summer NWP background mean state.…”

<bold>ENSO</bold>组合模态对夏季西北太平洋异常反气旋维持的关键作用

“…作 为 热 带 海 气 耦 合 年 际 变 率 的 最 主 要 模 态 ， 厄 尔 尼 诺 和 南 方 涛 动 (El Niño and Southern Oscillation, 简称 ENSO)对全球气候都能产生重要影响 (Bjerknes, 1969;Wallace and Gutzler, 1981;Wallace, et al, 1998;Alexander, et al, 2002) ，其中对东亚地区的气候影响 也非常显著 (Zhang, et al, 1996;陶诗言和张庆云, 1998;Chen, et al, 2000;Wang, et al, 2000) 。 研究表明，西北太平洋异常反气旋(WNPAC)/气旋(WNPC)是 ENSO 影响东亚气候的 重要纽带，它一般从 ENSO 发展年秋季开始建立，在衰减年春季达到鼎盛状态，并一直持 续至初夏 (Harrison and Larkin, 1996;Wang, et al, 2000;Zhang et al, 2017) 。在 El Niño 成熟 期，受 WNPAC 西北侧异常西南暖湿气流的影响，东亚冬季风容易偏弱 (Zhang, et al, 1996;Wang, et al, 2003) ，中国东南部降水容易偏多 (Zhang, et al, 1999(Zhang, et al, , 2002Wang, et al, 2017) 。在 El Niño 衰减年夏季，WNPAC 的位置有所北抬，我国长江中下游地区梅雨降水 增强 (Chang, et al, 2000a(Chang, et al, , 2000bLin Z D and Lu R Y, 2009;黄平和黄荣辉, 2010;Tim, et al, 2017;Zhang et al, 2017) 。当 La Niña 事件发生时，情况虽然并不是反对称，但大致相反 (Matsuno, 1966;Gill, 1980) ，其西部的东北风异常与气候态东北信风叠加，使 风速增强、向上的潜热通量增加，从而导致西太平洋地区的海温变冷进而在其西侧激发出 反气旋性环流，其东侧的偏北气流进一步使得信风增强，海温进一步变冷，由此使得 WNPAC 一直维持到 El Niño 衰减年的初夏。 然而，到 El Niño 衰减年夏季，WNP 局地冷海温的信号已经消失，这似乎意味着风-蒸 发-海温正反馈机制对夏季 WNPAC 作用较弱。随后，大量的研究开始从印度洋(IO)海温 的角度(称为印度洋电容器效应)解释夏季 WNPAC 的维持 (周天军等, 2004;黄刚和胡开 明, 2008;Wu, et al, 2010) ，也有研究强调了印度洋-西北太 平洋联合效应(Indo-western Pacific Ocean Capacitor, 简称 IPOC)对 WNPAC 维持的关键作 用 (Wang, et al, 2013;Xie, et al, 2016;Wang, et al, 2018Wang, et al, , 2020 。El Niño 衰减年 IO 暖海温 异常在春季达到峰值并一直持续到夏季 (Klein, et al, 1999) 。因此，ENSO 期间 WNPAC 形成和维持的物理机制仍存在一些亟 待解决的争议点。近年来，Stuecker 等(2013)在赤道太平洋低层环流场中发现并定义了 ENSO 组合模态(Combination Mode，简称 C-mode) ，该模态具有 10 和 15 个月左右的主周 期，在物理上来源于 ENSO 年际变率与海温年循环之间的非线性相互作用，在空间型上则 表现为中太平洋赤道南侧的西风异常和西北太平洋上的反气旋性环流 (McGre...…”